On This Day . . .

The Sky Comes Alive

August 28, 1859: Aurora Borealis Blazes Over USA & Europe

On this day in 1859 a geomagnetic storm causes the Aurora Borealis to shine so brightly that it is seen clearly over parts of USA, Europe, and even as far away as Japan.

The auroras that resulted from the “great geomagnetic storm” on both 28 August and 2 September 1859 are thought the most spectacular in recent recorded history. It was reported by the New York Times that in Boston on Friday 2 September 1859 the aurora was “so brilliant that at about one o’clock am ordinary print could be read by the light”.

The aurora is thought to have been produced by one of the most intense coronal mass ejections in history, very near the maximum intensity that the Sun is thought capable of producing. It is also notable for the fact that it is the first time where the phenomena of auroral activity and electricity were unambiguously linked.

As Seen From International Space Station

How Does This Happen

Auroras result from emissions of photons in the Earth’s upper atmosphere from ionized nitrogen atoms regaining an electron, and oxygen and nitrogen atoms returning from an excited state to ground state. They are ionized or excited by the collision of solar wind and magnetospheric particles being funneled down and accelerated along the Earth’s magnetic field lines; excitation energy is lost by the emission of a photon, or by collision with another atom or molecule:

Oxygen emissions

Green or brownish-red, depending on the amount of energy absorbed.

Nitrogen emissions

Blue or red; blue if the atom regains an electron after it has been ionized, red if returning to ground state from an excited state.

Oxygen is unusual in terms of its return to ground state: it can take three quarters of a second to emit green light and up to two minutes to emit red. Collisions with other atoms or molecules absorb the excitation energy and prevent emission.

Because the very top of the atmosphere has a higher percentage of oxygen and is sparsely distributed, such collisions are rare enough to allow time for oxygen to emit red. Collisions become more frequent progressing down into the atmosphere, so that red emissions do not have time to happen, and eventually even green light emissions are prevented.

This is why there is a color differential with altitude; at high altitude oxygen red dominates, then oxygen green and nitrogen blue/red, then finally nitrogen blue/red when collisions prevent oxygen from emitting anything. Green is the most common of all auroras. Behind it is pink, a mixture of light green and red, followed by pure red, yellow (a mixture of red and green), and lastly, pure blue.

Auroras are associated with the solar wind, a flow of ions continuously flowing outward from the Sun. The Earth’s magnetic field traps these particles, many of which travel toward the poles where they are accelerated toward Earth. Collisions between these ions and atmospheric atoms and molecules cause energy releases in the form of auroras appearing in large circles around the poles. Auroras are more frequent and brighter during the intense phase of the solar cycle when coronal mass ejections increase the intensity of the solar wind.